For many years, thermosetting phenolic resins have been molded using standardized compression or transfer molding techniques. While these techniques generally provide molded articles having excellent dimensional stability and good physical properties, technical improvements leading to cost reduction and increased productivity are required in order to enable thermosetting phenolics to remain competitive with other plastics and materials of constructions such as metals and ceramics. One such improvement has been the application of injection molding techniques to fabricate parts from thermosetting phenolic molding compositions. The injection molding process offers the advantages of reduced molding cycles, better control of process variables, and increased productivity as compared with conventional compression and transfer molding processes. The major disadvantage with the injection molding of thermosetting materials lies in the inevitable generation of a considerable amount of scrap, particularly when employing multiple cavity systems. This scrap represents thermosetting material that has cured (become infusible) in the runner and cannot be reused. The amount of non-reusable scrap generated in this fashion can be substantial, typically ranging anywhere from 15% to 80% of the total amount of material required to mold a part.
A more recent technical advance in the molding art has been the adaptation of the runnerless injection, or cold manifold, process to injection mold thermosetting phenolics. In the cold manifold process, the material in the sprue and manifold system (the so-called "runner") is maintained at a sufficient temperature to plasticize the material, without causing it to cure or "set-up" prematurely. Thus, when a cured part is removed from the mold cavity, the material in the sprue and manifold becomes part of the next molding, instead of being discarded as in conventional injection and transfer molding operations, the runnerless injection process, therefore, provides for significant savings in material, and, in addition, increased industrial efficiency by the elimination of secondary operations such as extra finishing and secondary gate grinding.
The thermosetting materials employed in runnerless injection processes differ, in certain respects, from materials normally employed in conventional injection processes due to the different requirements of each process. One significant difference is that a standard injection or transfer molding material typically has a stiffer plasticity for faster molding cycles. In contrast, a runnerless injection material should remain in a plasticized or fused condition in the manifold or barrel of the mold for extended periods of time without curing prematurely at the manifold temperature, i.e. usually about 125.degree. C., while being capable of curing rapidly in the mold cavity at the molding temperature, i.e. usually about 170.degree. C. In addition, the molded part should also have good dimensional stability and physical properties.
The prior art discloses various thermosetting compositions which are directed to runnerless injection applications. For instance, U.S. Pat. No. 3,959,433, to Sauers, discloses the addition of non-polymeric para-substituted phenols, such as Bisphenol-A, to a thermosetting phenolic resin in order to reduce the viscosity of the resin in the manifold, i.e. to improve its processibility. This composition is limited in terms of the range of monomer or dimer employed, generally being less than 35 parts by weight based on 100 parts by weight of resin, since introducing higher concentrations in the resin composition tends to adversely affect the physical properties of a molded article by decreasing the cross-linking density of the cured article. Moreover, this composition has not been found to be effective in significantly improving the processing stability of the resin at the manifold temperature. As this is a critical parameter in any runnerless injection molding process, it will readily be apparent that a continuing need exists for improved runnerless injection materials, and, in particular, for improved materials having enhanced processing stability.
While the phenolic resins of this invention are primarily useful in runnerless injection processes, where low temperature processing stability is a critical factor, they also find utility in more conventional molding processes such as injection molding, extrusion, and transfer molding processes where material savings can also be a significant factor.